Use Newton's second law to determine the acceleration being applied to the sled. There are three forces at work on the sled (its weight, the force normal to the ground, and friction) but two of them cancel, leaving friction as the only effective force. This vector is pointed in the opposite direction of the sled's movement, so if we take the direction of its movement to be the positive axis, we would find the acceleration due to the friction to be
[tex]\vec F_G+\vec F_N+\vec F_F=m\vec a\iff-12\,\mathrm N=(20\,\mathrm{kg})a\implies a=-0.6\,\dfrac{\rm m}{\mathrm s^2}[/tex]
Now we use the formula
[tex]{v_f}^2-{v_i}^2=2a(x_f-x_i)[/tex]
to find the distance it travels. The sled comes to a rest, so [tex]v_f=0[/tex], and let's take the starting position [tex]x_i=0[/tex] to be the origin. Then the distance traveled [tex]x_f-x_i=x_f[/tex] is
[tex]-\left(4.5\,\dfrac{\rm m}{\rm s}\right)^2=2\left(-0.6\,\dfrac{\rm m}{\mathrm s^2}\right)x_f\implies x_f\approx17\,\mathrm m[/tex]
